A majority of the large electric power plants in the United States are steam-cycle plants, wherein the spent steam is condensed by ambient water supplied from a source such as a river, cooling tower, lake, or pond. In the overwhelming majority of these plants, the water is supplied by up to four separate pumps operated in parallel, with each driven by its own high-horsepower electric motor. (For efficiency, such large motors and pumps are run at their optimum output and are not controlled or controllable as to output, except as to being either on or off.) Conventionally, at least two equal capacity pumps are employed, so that there will always be at least one in operating condition, and so that it is not necessary to completely shut down the plant in case of motor or pump breakdown.
Under certain conditions, it has been understood that not all of the pumps needed to be operated. For example, during winter in some areas, when the ambient water temperature was near freezing, the better operators would shut down one or more of the pumps, especially when the electrical generating load was low. Such actions have been heretofore largely based on subjective judgment of the staff operating the plant and thus prone to error, especially when conditions were far from clear. In these circumstances, the normal response of operators is to operate more pumps rather than less, as the operators are often busy with more immediate problems and more pressing duties in the operation of the power plant, and operating more pumps than needed is considered the lesser evil, in view of the possible loss of electrical generation that might result from operating less than was needed. Also, it is often not easy to predict the exact operating condition for reducing multi-pump operation, as this depends on factors such as scale build-up in condensers, cooling waterflow, tubesheet pluggage, and exhaust steam enthalpy, which will vary over time.